Ball screw

ABSTRACT

A ball screw is provided having a ball roller screw ( 1 ) and a screw nut ( 2 ) arranged thereon, whereby the balls ( 3 ) are arranged to endlessly circulate in a ball channel ( 6 ) defined by ball tracks ( 4, 5 ) on the ball roller screw ( 1 ) and the screw nut ( 2 ). The ball channel ( 6 ) includes a load section ( 7 ) wound about the ball roller screw ( 1 ), the beginning and end of which are endlessly connected by a load-free return section ( 8 ). A minimum of two and a maximum of four balls ( 3 ) are housed in a common ball cage ( 10, 22 ), the cage pockets ( 11 ) of which each house one of the balls ( 3 ), whereby the ball cage ( 10, 22 ) includes discs ( 12, 15 ) at the axial ends thereof and between adjacent balls ( 3 ), that engage the ball channels ( 4, 5 ), and provided on the sides thereof facing the balls ( 3 ) with a concave receptacle ( 14 ) for the balls ( 3 ).

BACKGROUND

The present invention relates to a ball screw. Such ball screws converta rotating motion into a linear motion.

From U.S. Pat. No. 6,176,149 and DE 100 44 447 A1, for example, a ballscrew has become known, with a ball roller screw and a screw nutarranged thereon. Balls are arranged so that they can circulateendlessly in an endless ball channel limited by helical, spiral balltracks of the ball roller screw and the screw nut. The ball channelcomprises a load section wound about the ball roller screw, whosebeginning and end are endlessly connected to each other by a load-freereturn section. In the ball screw disclosed by U.S. Pat. No. 6,176,149,a plurality of spacers having concave surfaces are arranged betweenadjoining balls to reduce friction between adjoining balls. However, inthis configuration, the individual spacers do not restrict the balls'movement along the ball channel and ball clogging can still occur. Inthe ball screw disclosed by DE 100 44 447 A1, two balls are housed in acommon ball cage, whose cage pockets each house one of the balls. Theball cage is formed of a flat, disc-shaped base with oval outercontours, in which two circular recesses form the pockets for the balls.The pocket walls are slightly concave and enclose a portion of the ballcircumference on both sides of an equatorial plane, so that the ballsare held in these pockets so that they cannot be lost. Due to theminimal wrap-around by means of the concave pocket walls, a pronounceddip by the balls in these cage pockets is set. This dip—or also pocketplay—could be reduced in that the ball cage and thus the ballwrap-around have a thicker configuration. However, thicker ball cagesare associated with other disadvantages: the ball cage extends along theball channel. In this extension direction, the ball cage has webs, whichare arranged parallel to the ball tracks but outside of these balltracks. The radial spacing between the outer periphery of the ballroller screw and the inner periphery of the screw nut must beconstructed at least as large as the thickness of this web, so that theball screw is functional. If the ball cage has a thicker construction,that is, if the web is also thicker, then this leads to the result thatthis radial spacing increases. The ball screw then has a largerstructure. Another problem presents itself in the region of the returnsection. In the return section, typically there is a groove, in whichthis web runs. If this web now has a thicker construction, then thegroove and thus the return section must have a larger construction.

SUMMARY

The objective of the present invention is to provide a ball screwaccording to the features of the preamble of claim 1, in which the ballplay in the ball cage is reduced, without causing the describeddisadvantages.

According to the invention, this objective is met in that the ball cagehas discs at its axial ends and between adjacent balls, each of whichengages in the ball tracks and are provided with a concave receptaclefor the balls on their sides facing the balls. Consequently, for theball screw according to the invention, these discs extend into the balltrack cross section. Thus, the ball wrap-around is increasedsignificantly. The ball play is simultaneously considerably reduced dueto the increased ball wrap-around. The radial spacing between the outerperiphery of the ball roller screw and the inner periphery of the screwnuts remains unchanged, because the cage web can also remain unchangedin its thickness.

Preferably, two balls are arranged in one ball cage. The invention islikewise suitable, however, in an advantageous manner for ball cageswith at most four pockets, in each of which is housed a ball so that itcannot be lost.

In the ball screw, there is a plurality of such ball cages arranged onebehind the other in a line. To guarantee that touching ball cages cannotbe interlocked, an improvement according to the invention provides thatthe discs lying on the ends of the ball cage are constructed on theiropposing sides as convex bumpers. Consequently, two adjacent ball cagesboth have these convex bumpers on their facing sides, which guaranteesan interlocking-free passage through the ball channel.

During a complete cycle of the ball cages through the ball channel,pivoting motions are made between adjacent ball cages. The convexbumpers of the two ball cages touch in an approximately point-shapedcontact. This contact point of the two bumpers wanders as a function ofa pivot position of the two ball cages relative to each other along theconvex bumper surface. These convex bumpers are adjusted to each otherso that an interlocking-free passage through the ball channel isguaranteed. The bumpers can also be adjusted to each other so thatduring the wandering of the contact point, there are no or onlynegligible relative motions in the direction of the ball channel betweenthe two ball cages.

The already mentioned web is arranged on at least one longitudinal sideof the ball cage, wherein all of the discs are connected by the web.Such ball cages are preferably made from plastic in an injection-moldingprocess. The plastic can preferably have a flexible, smooth,low-friction, and noise-damped construction. If only one web isprovided, the pockets can have an open construction and the ball cagebecomes even more flexible. That is, the balls are not enclosed over acomplete equatorial circumference. However, two webs can also beprovided, wherein the other web is arranged on the other longitudinalside. In this case, the balls are completely enclosed about an equator.

According to another improvement of the invention, the web can beprovided on sides facing away from each other with a guide cam forguiding the ball cage, wherein one side is arranged radially outside ofthe screw nut and the other side is arranged radially inside the ballroller screw. These guide cams become important especially in the returnsection. In the return section, the balls circulate without loading.Therefore, in the return section the balls can contact walls definingthe return section and can cause undesired noise. This noise can now beprevented such that, on one hand, the ball play is considerably reducedby the discs according to the invention and, on the other hand, thereturn section is provided with guide surfaces for guiding the cage web.In this way, in the return section, it can be guaranteed that the ballcage is guided exclusively on these guide surfaces. Consequently, theballs cannot contact the walls defining the return section despite asmall play in the ball cage, but instead pass through the return sectionwithout touching the walls.

These guide surfaces can be preferably constructed on facing groovewalls of a guide groove.

For reducing the friction between the balls and the ball cage, a rollerbody, which is in rolling contact with the two balls of a ball cage, canbe arranged between the two balls. Preferably, the rotational axis ofthis intermediate roller body lies in a common plane with the rotationalaxes of the balls.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in more detail below with reference to threeembodiments shown in a total of eleven figures. Shown are:

FIG. 1 a partial cross sectional view through a ball screw according tothe invention,

FIG. 2 balls arranged in ball cages for the ball screw according to theinvention,

FIG. 3 the ball cage shown in FIG. 2 in the return section of the ballscrew according to the invention,

FIG. 4 a view of the return section shown in FIG. 3,

FIG. 5 a perspective view of an individual ball cage of the ball screwaccording to the invention,

FIG. 6 the ball cage from FIG. 5 in a longitudinal section view,

FIG. 7 the ball cage according to FIG. 5 in another longitudinal sectionview,

FIG. 8 a perspective view of a modified ball cage of another ball screwaccording to the invention,

FIG. 9 a partial cross-sectional perspective view through the returnsection according to FIG. 3,

FIG. 10 a top view on the longitudinal section according to FIG. 9,

FIG. 11 a partial section in the return section of another ball screwaccording to the invention, and

FIG. 12 a cross-sectional view of a prior art ball screw.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The ball screw according to the invention shown in FIGS. 1 to 7 andFIGS. 9 and 10 has a ball roller screw 1 and a rotatable screw nut 2arranged thereon. Balls 3 are arranged so that they can circulateendlessly in an endless ball channel 6 limited by ball tracks 4, 5. Theball channel 6 has a load section 7, which is wound about the ballroller screw 1 and in which the balls 3 are arranged on the ball tracks4, 5 so that they can roll under loading. The beginning and end of theload section 7 are connected to each other endlessly by a load-freereturn section 8. The return section 8 is here constructed on alongitudinally divided tube 9, whose one half is shown in FIG. 3.Furthermore, there is a plurality of ball cages 10 arranged one behindthe other in the ball channel 6. In each ball cage 10, two balls 3 arehoused in ball pockets 11 so that they cannot be lost. Several of theseball cages 10 arranged one behind the other are shown in FIG. 2.

FIG. 5 shows in perspective view one of these ball cages 10. The ballcage 10 has a disc 12 on each of its two axial ends. These two discs 12are convex on their sides facing away from each other and form bumpers13. The sides of these discs 12 facing each other are concave and formreceptacles 14 for the balls 3. The discs 12 enclose with thereceptacles 14 a portion of the ball periphery on both sides of anequatorial plane, so that the balls 3 are held in the cage pockets 11with little play so that they cannot be lost. Another disc 15 isarranged between the two cage pockets 11 of the ball cage 10. The sidesof the discs 15 facing the discs 12 are likewise concave and formreceptacles 16 for the balls 3.

The bumpers and the receptacles are to be seen especially clearly inFIG. 6.

Both longitudinal sides of the ball cage 10 are formed by webs 17, whichconnect the discs 12, 15 to each other. The ball cage 10 is presentlymade from plastic in an injection-molding process, wherein the webs 17and the discs 12, are integrally connected to each other.

It can be noted from FIG. 1 that the discs—here discs 12—fill up themajority of the ball channel 6. The discs 12, 15 thus engage in the balltracks 4, 5 of the ball roller screw 1 and the screw nut 2. It can befurther inferred from FIG. 1 that the webs 17 have a very flatconstruction in comparison with the discs 12, 15. The ball cage 10 isguided in the load section 7 via the webs 17 on the inner periphery ofthe screw nut 2 and on the outer periphery of the ball roller screw 1.

It can be noted from FIGS. 3, 5, 6 that each web 17 is provided with twoguide cams 18 on web sides facing away from each other. These guide cams18 are used for guiding the ball cage 10 in the ball channel 6. Theguide cams 18 gain special meaning in the return section 8.

FIG. 9 shows in perspective view a partial section through the returnsection 8. In the return section 8 there are two guide grooves 19, ofwhich one is shown. Facing groove walls 20 of the guide groove 19 formguide surfaces 21 for the ball cage 10. It can be noted from FIG. 9 thatthe web 17 engages with its guide cams 18 in the guide groove 19. Theguide cams 18 contact the guide surfaces 21 of the guide groove 19. Inthe return section 8, the ball cage 10 is guided without a problem. Theball channel 6 is provided within the return section 8 with a diameterthat is sufficiently large that the balls 3 are not in contact with thetube 9. The enclosing of the balls 3 with the receptacles 14, 16 of thediscs 12, 15 over a large ball periphery section guarantees only littleplay or ball dip in the ball cage 10. In this way, rattling noises inthe region of the return section are ruled out due to the absence of theballs striking the walls of the return section.

FIG. 8 shows a modified ball cage 22. This differs from the ball cagedescribed farther above in that instead of two webs only one web 17 isconstructed. Consequently, it is sufficient to construct only one guidegroove 19 in the return section 8.

It can be noted from FIG. 10 that in the return section 8, the balls 3are arranged with a spacing relative to the tube wall, which defines theball channel 6 in the region of the return section.

FIG. 11 shows in a longitudinal section like that of FIG. 10, a sectionof another ball screw according to the invention. In contrast to theball screw according to the invention described previously, here thedisc 15 is provided with a pocket 23, in which a roller body 24 isarranged so that it can rotate. This roller body 24 stands in rollingcontact with the two adjacent balls 3. The rotational axes of the balls3 and the roller body 24 lie in a common plane. In this way, the slidingfriction of the balls 3 is reduced with the ball cage 10.

REFERENCE NUMBERS

-   1 Ball screw-   2 Screw nut-   3 Ball-   4 Ball track-   5 Ball track-   6 Ball channel-   7 Load section-   8 Return section-   9 Tube-   10 Ball cage-   11 Cage pocket-   12 Disc-   13 Bumper-   14 Receptacle-   15 Disc-   16 Receptacle-   17 Web-   18 Guide cam-   19 Guide groove-   20 Guide wall-   21 Guide surface-   22 Ball cage-   23 Pocket-   24 Roller body

1. Ball screw comprising a ball roller screw and a screw nut arrangedthereon, wherein balls are arranged to circulate endlessly in an endlessball channel limited by ball tracks of the ball roller screw and thescrew nut, the ball channel has a load section, which is wound about theball roller screw and has a beginning and end that are connected to eachother endlessly by a load-free return section, a plurality of commonball cages, each housing at least two and at most four balls in separatecage pockets that each house one of the balls, the ball cages have discswhich engage in the ball tracks at axial ends thereof and betweenadjacent ones of the balls, the discs include a concave receptacle forthe balls on sides thereof facing the balls, and two of the discslocated at the ends of the ball cages include three-dimensionallypartially ball-shaped convex bumpers on sides thereof facing away fromeach other, each of the bumpers is adjacent to one of the bumpers at anend of another one of the ball cages during operation.
 2. Ball screwaccording to claim 1, wherein a web, which connects all of the discs toeach other, is arranged on at least one longitudinal side of the ballcage.
 3. Ball screw according to claim 2, wherein the ball cage isproduced from plastic in an injection-molding process.
 4. Ball screwaccording to claim 2, wherein the web is provided with a guide cam forguiding the ball cage located on each side thereof facing away from eachother.
 5. Ball screw according to claim 2, wherein the screw nut isprovided in the return section that extends along the ball tracks withguide grooves for guiding the ball cage, and the cage web is arrangedbetween the guide surfaces of the guide grooves.
 6. Ball screw accordingto claim 5, wherein the guide surfaces are constructed on facing groovewalls of the guide groove.
 7. Ball screw according to claim 5, whereinin the return section, the balls are held in the cage pockets of theball cages and do not contact the ball channel.
 8. Ball screw accordingto claim 1, wherein a roller body, which is in rolling contact with thetwo balls, is arranged in a pocket defined in the disc between the ballsin the ball cage.
 9. Ball screw according to claim 8, wherein arotational axis of the roller body is arranged in a common plane withrotational axes of the balls.